The linear equation

Linear motors are becoming an alternative to traditional ballscrews for machine tools’ drives, writes Anna Kochan

BMW’s decision last year to introduce machine tools fitted with linear motors in one of its machine shops was a breakthrough in the adoption of this technology. Despite their attractions – faster machining, with an acceleration of higher than 1g – the number of machine tools driven by linear motors in industry is small.

However, as this year’s EMO European machine tool show in Paris revealed, firms are developing the technology despite low sales, and new entrants are not being put off.

BMW opted for linear motors after six months of trials, during which four machining alternatives were assessed. The technologies were linear motor, conventional ballscrew, high-speed ballscrew and ballscrew with two asynchronous spindles. The comparison was based on the machining of a cylinder head. Lutz Schaufuss, project engineer in BMW’s engine machine shop in Munich, says results favoured the linear motor machine: `It wins hands down on machining times and reduces floor space and maintenance needs.’

Machines in the Munich shop, which produces small batches of top-of-the-range engines for BMW and Rolls-Royce, were 12 years old. This, plus a wish to increase special engine output from 15,000 to 30,000 per year, led the company to invest in new machines. `We were seeking flexible production capability, high precision and advanced machine technology,’ Schaufuss says.

BMW installed nine linear-motor driven Urane 25 machining centres from Renault Automation, which are now machining aluminium cylinder heads and cylinder blocks. The Urane 25 is a third more expensive than an equivalent ballscrew-driven machining centre, but savings in machining cycle times compensate. Cycle time savings were around 50-80%, Schaufuss says.

The machine performed with a positioning accuracy of 4 micro m and a repeatability of 2 micro m during the test period, better than the ballscrew-driven machines by a factor of two.

Schaufuss plans to buy another 10 or 11 linear motor-driven machines soon. His team is doing machining tests on steel and cast-iron parts.

Renault Automation has had orders for flexible machining lines incorporating 81 Uranes from car maker Renault and from Peugeot-Citroen for sites in France and Spain. The Renault systems will be capable of machining 1,400 8- and 16-valve cylinder heads daily.

But a boom is not yet in sight, says linear motor manufacturer Krauss Maffei. `Market growth has not yet met expectations and the machine tool industry still appears cautious,’ says Thorsten Rabenschlag, deputy sales manager.

A problem is that when machining ferrous materials, a linear motor’s magnetism attracts ferrous chips. Air jets can keep chips away from the motor, says Rabenschlag. Krauss Maffei’s machine tool design limits the motor’s magnetic attraction to 50mm around the moving component.

To exploit the high speeds of linear motors, it is best to use them with light alloys, adds Horst Stratmann, sales manager at German machine tool firm Huller Hille.

Ex-Cell-O introduced linear motor technology in 1993, and says it has sold more than 100 machines. According to Sales manager Hilmar Rudlof, the main buyers have been automotive industry companies, for machining aluminium components, with a few machining cast iron. `With aluminium, the reason for going to linear motors is reducing non-cutting times. With cast iron, the main consideration is accuracy. Machines with linear motors provide an accuracy which is better by a factor of two than those with ballscrews,’ he says. The magnetism problem has been solved by arranging the motors on the sides of the machine, not beneath the spindle where the chips fall.

But Rabenschlag at Krauss Maffei believes linear motors have more potential in the fields of material handling equipment and robotics. `High dynamics is increasingly required in automation and robotics. This is the growing area,’ he says. `We have introduced a lower specification of linear drive which is 30% less expensive than the current range because the same levels of precision are not required. They will start to replace rack-and-pinion drives, ballscrews and belts for strokes varying from a few centimetres up to 10 metres.’